The invention relates to a combination of a screw, with a shaft and a head, having a circular conical sealing disk that is guided or can be guided on the shaft up to the bottom of the screw head.
Such combinations of a screw with a conical sealing disk are known, for example, from the company brochure topform screw fastener: Technical Specification (Technische Dokumentation), SFS Stadler Befestigungstechnik Bau, CH 9435 Heerbrugg, BB.3.27.88.D, 1986. On the screw of this combination, the circular bottom of the screw head has a surface perpendicular in reference to the axis of the screw. Such combinations serve to fasten components to a thin steel or aluminum substructure or generally for fastening sheet metal to sheet metal or sheet metal to wood. A screw as used in the above-mentioned company brochure is also shown in U.S. Pat. No. 6,854,942 B1 differing from the screw shown in the brochure only in that below the screw head a special additional thread is provided on the shaft, by which the sealing disk is held below the screw head perpendicularly in reference to the axis of the shaft.
Prior art also includes combinations of a screw in which the bottom of the head is embodied concavely so that a rubber gasket can be held more securely between the screw head and the support. Such combinations of a screw with a rubber gasket are known, for example form U.S. Pat. No. 4,875,818, EP 140991 B1, or U.S. Pat. No. 6,789,989 B2. A tapping screw with a concave bottom of the screw head is known from EP 0243526 B1. By the concave embodiment of the bottom of the screw head space is created to accept a circular bead, which is provided around a receiving bore in a cover sheet or the like, into which the screw can be driven.
Combinations of the type of screw and circular conical gasket of the type mentioned at the outset is preferably used for the assembly of profiled steel sheets for roof, wall, and ceiling construction, such as known e.g., from the IFBSINFO guideline 8.01 by the Industrieverband zur Foerderung des Bauens mit Stahlblech e.V. (Industrial Association for Promoting Construction with Steel Sheet-metal), Issue April 2002. According to the brochure, page 10, the gaskets comprise a metal back onto which an elastomer layer made from EPDM is vulcanized in an undetachable manner. The metal back is usually 1.0 mm thick and comprises stainless steel, aluminum, or galvanized steel. The elastomer layer EPDM is 2.0 or 3.0 mm thick. When the screws are tightened, these gaskets form an interior seal at the shaft of the screw and an exterior seal at the point where the movements of sheet metal or the like have to be compensated. Temperature related expansions of the structural elements and traffic load of a building are compensated according to specifications. The large sealing surface simultaneously prevents any metal/metal contact (galvanic corrosion) and protects the components from mechanic damage by assembly tools. This can only work flawlessly when the combination of screw and gasket has been correctly placed with a precisely adjusted depth of insertion (path-related screw devices.) The gaskets are assembled on the screws such that they cannot be lost. The screws reach the construction site ready for use as a screw/gasket combination. In this context, the above-mentioned guideline requires (at page 25) that screws with gaskets are to be screwed in according to a depth stop. Any improper deformation of the gasket (washer with elastomer seal) shall be prevented by the depth stop on the screw. The screw may only be tightened to such an extent that the washer and the elastomer seal are compressed according to the specifications of the manufacturer. Unfortunately, in practice such screws with gaskets are used without any depth stop, contrary to the guideline. This leads to the screws being screwed-in either too loosely or too tightly. In the above-mentioned company brochure this is shown on page 10 and in the above-mentioned guideline on page 25.
A particular problem is given when the screw is over-tightened. This can be clearly illustrated in the combination of screw and gasket shown in the attached FIG. 5 (prior art.) In FIG. 5 the screw in its entirety is marked with the reference character 10 and the gasket in its entirety with the reference character 20. According to the illustration in FIG. 5b, the gasket 20 is guided on a shaft 12 of the screw to the bottom 14′ of the screw head 16 and comes into a linear contact with the bottom 14′ at a circle at the interior perimeter of the circular gasket 20. In this position the gasket 20 is held on the shaft 12 of the screw 10 by an elastomer seal 22.
When placing the screw 10, based on the linear contact between the screw head 16 and the gasket 20, first pressure is applied to the gasket in the proximity of its center. Then, starting from this linear contact, increasingly a circular planar contact develops between the screw head 16 and the gasket 20.
When installation occurs without any depth stop, unfortunately a common practice, the pressure of the screw head 16 acting upon the gasket 20 causes the gasket to be finally brought into a planar shape from which it further converts into a conical shape in a snapping motion, with its cone tapering downwards, contrary to the original cone which was tapering upwards. This process is called “cupping”, because the gasket accepts a cup-shape. In this position of the gasket 20 a tight connection is not ensured because the screw head 16 contacts the upper side of the gasket 20 only at the exterior perimeter of its collar 17 and because the elastomer seal of the gasket has been improperly deformed such that it bulges out underneath the gasket and no longer sufficiently seals the exterior perimeter of the shaft. Water will collect in the cup-shaped gasket and finally finds its way to below the screw head and extends along the shaft downwards, because as already stated the elastomer seal no longer seals properly at the shaft.
It is particularly problematic that the force to be applied maximally to the screw head is almost precisely equivalent to the one at which a trend for cupping develops. For this trend, the existence of an elastomer seal 22 between the support and the gasket 20 is of major importance. The displacement of the elastomer seal 22 beyond the exterior perimeter of the gasket 20 enhances the trend for cupping even more, because the support at the interior perimeter of the gasket 20 becomes correspondingly smaller.
U.S. Pat. No. 4,292,876 states that in a combination of a screw and a gasket the trend for “cupping” can be reduced by the use of a conical gasket comprising a steel ring and an elastomer seal, able to withstand higher torque.
In EP 0760431 A1 problems are discussed which occur when in a combination of a screw and a rubber gasket either insufficient or excessive pressure is applied to the rubber gasket when the screw is placed. As one solution for this problems it is suggested to adjust the torque or the depth stop of the placement tool such that a certain pressure to the rubber gasket is not exceeded. It is suggested in another solution to hold the rubber gasket back by way of flanges.
From U.S. Pat. No. 5,156,509 it is known to ensure tightness in a combination of a screw and a gasket comprising a metal part and an elastic seal such that the metal part and the seal are arranged in a recess.
Finally, from U.S. Pat. No. 2,982,573 it is known to use a metal gasket, which in its original state is conical, made from metal and a rubber seal mounted thereto, which are compressed flat when the screw is tightened. For achieving a better seal the metallic gasket has a flange at its exterior perimeter by which the rubber seal is held back when the metallic gasket is compressed flat.